The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and evolved UTRAN (E-UTRAN) is the radio access network of an LTE system.
As illustrated in FIG. 1, an E-UTRAN typically comprises user equipments (UE) 150a, 150b wirelessly connected to radio base stations (RBS) 100a, 100b, 100c commonly referred to as eNodeBs (eNB).
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system such as the one illustrated in FIG. 1. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas. This results in a multiple-input multiple-output (MIMO) radio communication channel and such systems and related techniques are commonly referred to as MIMO. Several wireless standards nowadays support MIMO antenna deployments and MIMO related techniques such as spatial multiplexing, beam-forming and diversity.
In E-UTRAN, spatial multiplexing is used in the downlink. With spatial multiplexing it is possible to simultaneously transmit on several layers when the channel conditions are good. When channel conditions are bad, the multiple antennas may be used for beam-forming instead. The channel capacity can thus under favorable channel conditions grow with the number of antennas.
Spatial multiplexing and beam-forming is combined with either channel dependent, or channel independent pre-coding, also referred to as respectively closed-loop and open-loop pre-coding. The pre-coding serves two purposes. When the number of signals to be spatially multiplexed equals the number of transmit antennas, the pre-coding can be used to orthogonalize the parallel transmissions, allowing for improved signal isolation at the receiver side through reduced inter-layer interference. When the number of signals to be spatially multiplexed is less than the number of transmit antennas, the pre-coding in addition provides the mapping of the signals to the transmit antennas.
With e.g. two transmitting antennas at the eNB and two receiving antennas at the UE, two layers are possible in the downlink. This means that two signals may be transmitted simultaneously when channel conditions are good. This is referred to as a rank 2 channel. If channel conditions get worse, only one signal will be transmitted (rank 1).
In a wireless communication system, such as the E-UTRAN illustrated in FIG. 1, it is desirable to reuse as much of the radio resources in each cell 110a, 110b, 110c as possible to achieve good spectral efficiency. Spatial multiplexing and frequency reuse are both a kind of reuse of resources. However, whenever a resource is reused this may lead to interference (intra-cell or inter-cell interference) between the links utilizing this particular resource.
Various approaches to manage the interference are known. One approach relies on actively selecting which UEs that can access a particular resource based on channel state information for these UEs. Of all possible UEs, those who interfere the least with each other may be scheduled jointly. Another approach is to use information about eNB and UE antenna polarization when allocating radio resources to the UEs, as the isolation is good (i.e. the interference is low) between orthogonally polarized transmissions.
The benefit of using antenna polarization information when scheduling radio resources to UEs is greatest when the transmissions to and from UEs are primarily either vertically polarized (VP) or horizontally polarized (HP). The reason for this is that these polarizations are well preserved in the wireless propagation channel, even in heavily shadowed situations. A transmitted vertically polarized wave will thus keep its vertical polarization throughout the propagation to a receiving side with a very limited cross-polarization scattering, and the corresponding is true for a horizontally polarized wave. In contrast, polarizations that contain elements of both VP and HP are not as well preserved and will therefore be less useful.
In a 2×2 (two transmitting and two receiving antennas) MIMO system using spatial multiplexing transmission mode, a cross polarized eNB antenna configuration with VP and HP antennas is therefore a good alternative. This would enable a VP transmission on one layer and a HP transmission on the second layer, which would thus minimize interference between layers.
Furthermore, due to the limited amount of cross-polarization scattering in the radio channel, the vertical-to-vertical and horizontal-to-horizontal polarization combinations (transmitting antenna and receiving antenna polarization combination) provide an equal received signal power on average, while the cross-polarized combinations (vertical-to-horizontal or vice versa) typically provide 5-15 dB weaker received signal power. A signal transmitted by an eNB with a VP transmission antenna will thus be received as a stronger signal in a UE with a VP receiving antenna (i.e. a vertical-to-vertical polarization combination) than in a HP receiving antenna (vertical-to-horizontal polarization combination).
How antenna polarization information may be used when scheduling radio resources to UEs, also referred to as polarization-based interference management, is in the following described with reference to FIG. 2. A UE 260 with a VP antenna would benefit from being scheduled to transmit to or receive from RBSs 200 utilizing a transmission mode that results in a VP channel 220 (vertical-to-vertical polarization combination), jointly with a second UE 250 that has a horizontal HP antenna and that is scheduled to RBSs utilizing a HP transmission mode resulting in a HP channel 210. The interference suppression between the two UEs will be an additional 5-15 dB compared to if the two UEs would both have been using the same polarization combination (i.e. non-orthogonal polarization).
In order to use polarization-based interference management, there is thus a need to assure that an eNB transmits VP and HP signals. One straight forward solution to assure this is to use VP/HP-polarized antenna configurations. However, there are drawbacks with using this kind of antenna configuration, and the overwhelming majority of existing sites with dual-polarized antennas therefore utilize slant +45/−45 polarized antennas.
There are several reasons why slant +45/−45 polarized antennas are preferred. One reason is that this antenna configuration provides symmetry of the radiation patterns. Another reason is that such an antenna configuration generates polarization orthogonality away from boresight, i.e. in all directions. Yet another reason has to do with power amplifier (PA) utilization. As there is one PA for each antenna, it is preferable to transmit a signal using both antennas, as the output power then can be doubled compared to if only one antenna and one PA is used. With a slant +45/−45 polarized antenna configuration, a VP or HP transmission would use both antennas and both PAs. With a VP/HP antenna configuration, a VP or HP transmission would use only one antenna and PA, thus leading to less output power.
Regardless of what cross-polarized antenna configurations that are used, it would be beneficial to adjust the absolute polarization of transmitted signals to a VP or HP transmission, in order to enable good isolation between the VP and HP transmitted signals, and to gain the full potential of polarization-based scheduling. However, the only way to detect if it is VP and HP that is actually transmitted over the air, is to introduce external calibration equipment to detect and adjust the absolute polarization, which is a costly solution.